test(config): env wins over config cli on nonmergeable list

This is a showcase of a bug. Will fix in the next commit

Author: weihanglo

tests/testsuite/config.rs

@@ -2224,6 +2224,36 @@ credential-provider = ['c', 'd']
     // expect: no merge happens; config CLI takes precedence
     assert_eq!(provider.path.raw_value(), "cli");
     assert_eq!(provider.args, ["cli-arg"]);
+
+    let env = "CARGO_REGISTRIES_EXAMPLE_CREDENTIAL_PROVIDER";
+    let gctx = GlobalContextBuilder::new()
+        .env(env, "env env-arg")
+        .cwd("foo")
+        .build();
+    let provider = gctx
+        .get::<Option<RegistryConfig>>(&format!("registries.example"))
+        .unwrap()
+        .unwrap()
+        .credential_provider
+        .unwrap();
+    // expect: no merge happens; env takes precedence over files
+    assert_eq!(provider.path.raw_value(), "env");
+    assert_eq!(provider.args, ["env-arg"]);
+
+    let gctx = GlobalContextBuilder::new()
+        .env(env, "env env-arg")
+        .config_arg(cli_arg)
+        .cwd("foo")
+        .build();
+    let provider = gctx
+        .get::<Option<RegistryConfig>>(&format!("registries.example"))
+        .unwrap()
+        .unwrap()
+        .credential_provider
+        .unwrap();
+    // expect: no merge happens; cli takes precedence over files and env
+    assert_eq!(provider.path.raw_value(), "env");
+    assert_eq!(provider.args, ["env-arg"]);
 }
 
 #[cargo_test]